The problems of corrosion and scale formation and attendant effects have troubled water systems for years. For instance, scale tends to accumulate on internal walls of various water systems such as boiler and cooling systems and thereby materially lessen the operational efficiency of the system.
Deposits in lines, heat exchange equipment, etc., may originate from several causes. For example, precipitation of calcium and magnesium salts and metal oxides in the water system leads to an agglomeration of the scale imparting compounds along or around the metal surfaces which contact the flowing water circulating through the system. In this manner, heat transfer functions of the particular system are severely impeded.
Corrosion, on the other hand, is a degrative electrochemical reaction of a metal with its environment. Simply stated, it is the reversion of refined metals to their natural state. For example, iron ore is iron oxide, iron oxide is refined to steel, when steel corrodes it forms iron oxide which, if left unattended, may result in failure or destruction of the metal. This can cause a particular water system to be shut down until the necessary repairs can be made.
As detailed in the Betz Handbook of Industrial Water Conditioning, 8th Edition, 1980, Betz Laboratories, Inc., Trevose, Pa., pages 85-96, the formation of scale and sludge deposits on boiler heating surfaces is a serious problem encountered in steam generation. Current industrial steam producing systems make use of sophisticated external treatment of the boiler feedwater such as coagulation, filtration, softening, etc. These operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge, silts and hardness imparting ions escape treatment and eventually are introduced into the stream generating steam.
In addition to the problems caused by mud, sludge or silts, the industry also has to contend with boiler scale. Although external treatment is utilized specifically in an attempt to remove scale imparting materials from the feedwater, scale formation due to residual hardness such as calcium and magnesium salts is always experienced. Accordingly, internal treatment, i.e., treatment of the water fed to the system is necessary to prevent, reduce and/or retard formation of scale imparting compounds and their deposition. The carbonates of magnesium and calcium are not the only problem compounds as regards scale. Waters having high contents of phosphate, sulfate and silicate either occurring naturally or added for other purposes can cause problems. Calcium and magnesium and any iron or copper present can react and deposit as boiler scale. As is obvious, the deposition of scale on the structural parts of a steam generating system causes poor circulation and lower heat transfer capacity resulting accordingly in an overall loss in efficiency. The use of isopropenyl phosphonic acid polymers and copolymers is described in U.S. Pat. Nos. 4,446,028 and 4,446,046. The ionic phosphonate monomer disclosed in these patents was found to be particularly useful in boiler systems where high temperatures and pressures were well suited to phosphonate chemistry treatment. The monomer unit was polymerized as a homopolymer or with anionic monomer units such as hydroxypropylacrylate.